1. Field of the Invention
The present invention is chiefly concerned with processes for increasing the chemical and biological reactivity of cellulose and/or hemicellulose in animal feedstuff materials. More particularly the invention relates to processes for (1) increasing the digestibility of cellulose-containing feedstuffs such as alfalfa press cake, aspen chips, barley straw, corn stover, rice straw, kochia stems, sweet sorghum, wheat chaff, wheat straw, and the like, by ruminant animals and (2) increasing the extraction of protein from within the cells which make up such animal feedstuff materials. This invention is also concerned with processes for increasing the water holding capacity of cellulose-containing materials and with processes for preparing cellulose-containing materials to undergo fermentation.
2. Description of the Invention
Many techniques have been used to increase the chemical and biochemical reactivity of cellulose. A number of factors influence this reactivity. They include particle size and fiber surface area, lignin content, cellulose crystallinity, etc. The prior art techniques used to accomplish this generally can be characterized as either physical or chemical in nature. The physical treatments include ball-milling to very small mesh sizes, two-roll milling and attrition milling. These physical treatments are effective in producing a highly reactive and accessible cellulose component from the material. However, the power requirements for size reduction are so large that such treatments are generally considered prohibitively costly.
Chemical treatments on the other hand usually involve the use of strong bases such as sodium hydroxide, strong acids, and various other cellulose swelling and dissolving agents such as certain transition metal complex cellulose solvents. Such chemical agents are expensive and therefore must be recovered to make these processes economically viable. Furthermore, many of these chemicals are toxic to or otherwise inhibit the biological processes associated with alcohol fermentation or the digestive processes of ruminant animals such as cattle, sheep, etc. Therefore, removal of these chemical agents from the treated cellulose-containing material must be very complete. Such requirements further increase the expense of these prior art chemical treatments. Furthermore, most prior art processes remove their chemical agents by washing with water. This has two major disadvantages. First, water dilutes the cellulose-swelling or dissolving agent to such an extent that the agent can no longer effectively swell the cellulose components of the materials. Hence these chemical agents must be concentrated for reuse; this generally involves considerable capital equipment and operating expense. Second, water is known to promote the recrystallization of decrystallized cellulose. Therefore any cellulose which does recrystallize becomes less reactive. See for example, Millet et al., "Pretreatments to Enhance Chemical, Enzymatic, and Microbiological Attack of Cellulosic Materials," Biotechnol. & Bioeng. Symp. No. 5, 193-219 (1975); and Millet, et al., "Physical and Chemical Pretreatment for Enhancing Cellulose Saccharification," Biotechnol. & Bioeng. Symp. No. 6, 125-153 (1976). Generally, these references discuss various prior art pretreatments of cellulosic materials such as woods and straw, to upgrade their digestibility. These pretreatments include mixing swelling agents with alkaline agents such as sodium hydroxide, primary amines, aqueous or gaseous ammonia and anhydrous ammonia in both liquid and gaseous forms. They also discuss various delignification processes such as the steaming of wood or straw in order to improve digestibility. They also discuss the effects of various physical treatments such as grinding, milling, irradiation, heating and/or compression.
Ammonia is a preferred agent in many of these prior art processes. By way of example, German Patent 169,880 ("the -880 patent") teaches use of relatively low levels of aqueous ammonia (8%), under pressure, to chemically attack lignin containing materials such as straw and wood. The results of chemical attacks of this kind may be one ore more of the following: (1) the addition of nitrogen from the ammonia ("nonprotein nitrogen") to the lignin-containing material for conversion to protein by ruminant animals, (2) the break-up of the lignin material found between the cellulose covered cells of these materials in order to produce products having more amorphous fiber structures and (3) the production of a slow hydrolysis of the material's cellulose components to sugars such as glucose.
Regardless of their end products however, chemical processes like those disclosed in the -880 patent are rather slow; ten hour process times are not uncommon. Moreover, little or no physical attack, as opposed to chemical attack, upon the cellulose cell walls is usually involved. For example, research done after the -880 patent reference disclosure, indicates that an 8% ammonia solution does not produce a physical swelling action upon cellulosic materials of this kind. See for example, Tarkow and Feist, Advances in Chem., Ser. 95, Amer. Chem. Soc., Washington, D.C. Other workers, e.g., Millet and Baker, Biotechnol & Bioeng. Symp. No. 5, 193-219 (1975) have found that the mechanism of digestion of aspen wood by liquid anhydrous ammonia is one of ammonolysis of cross-links of the glucuronic acid ester component of the lignin. This reference specifically notes that in an attempt to exploit this particular ammonolysis mechanism, a number of woods were exposed to anhydrous ammonia in both liquid and gaseous form and that the results were assay ed in terms of changes in in vitro digestibility. These workers found that Aspen appeared to be unique in the extent of its response to ammoniation. It attained a digestibility coefficient of about 50% as contrasted with Sitka spruce and red oak whose ammoniated product digestibilities were only 2 and 10% respectively.
Given these experimental results it appeared that "swelling" per se did not produce improved animal feeds. If this were the case, presumably woods other than aspen also would have become more digestible. Therefore, on balance these references seemed to indicate that ammonia takes part in a chemical attack upon the lignin which holds the cellulose crystalline structure in the materials employed in a given stereoscopic orientation but does not attack the cellulose cell walls of such materials. In other words the "glue" that holds the fibers together appeared to be weakened. However, these processes did not appear to attack the individual cells which make up the fibers. In other words, the cellulose coverings of such cells seem to be, for the most part, left physically intact even though the prior art swelling processes may have succeeded in splitting one set of cellulose fibers (chains of cellulose cells) from other cellulose fibers. In other words, these prior art processes tend to leave the basic unit of the fibers--the cells--intact.
Similarly, the use of ammonia as a swelling agent has been used in other kinds of chemical processes involving cellulosic materials. For example, U.S. Pat. No. 3,707,436 ("the -436 patent") teaches a process for manufacturing paper pulp by (1) impregnating wood chips with anhydrous ammonia, (2) heating the impregnated wood chips within an enclosed space under pressure and (3) suddenly releasing the pressure to cause deformation and disintegration of the chips to a fibrous condition in which the fibers are flexible, kinked, twisted and curled and therefore more susceptible to fibrillation by those mechanical procedures which are applied later in the paper making process. The fibers made by this process are specifically designed for paper making; hence they are characterized by attributes, e.g., possession of relatively light colors, possession of substantially all of the cellulose of the wood chips and susceptibility to delignification and bleaching to a high degree of brightness, attributes which are desired in the paper making field. Moreover, with respect to the subject of ammonia impregnation of the wood chips, the -436 patent teaches that the cellulose fibers can be rendered more plastic, apparently because the disclosed treatment strikes at the lignin glue which holds the cells together and thereby rendering the cellulose contained in the wood somewhat more amorphous and thus somewhat more plastic. This patent also teaches that such plasticity is retained by the product even after the nitrogenous agent is removed. This attribute demonstrates itself particularly well in the presence of heat, hot water and the like in this pulping procedure. This reference does not however concern itself with changing the reactivity of the cellulose for the purpose of enhanced digestibility by animals or for increasing the protein availability of the materials. Furthermore, it makes no suggestion of an increase in the glucose yield upon hydrolysis of the wood chips. This is not surprising since the fibers used in paper pulp operations are not intended to be converted into glucose.
Another treatment which involves both chemical and physical aspects is the so-called Masonite process wherein wood chips are saturated with water under pressure at about 300.degree.-500.degree. F. When the pressure is released, the water evaporates rapidly and the wood fibers tend to separate. This process is similar to what occurs when the moisture inside a popcorn kernel evaporates violently and the kernel greatly expands in volume. The Masonite process is effective in many situations; it does however require considerable amounts of thermal energy in the form of steam. It also has another disadvantage in that some of the plant material is inevitably degraded and made less useful by the high temperatures required. Finally, the moisture content of its resulting products is quite high, on the order of about 50%. See generally U.S. Pat. No. 3,707,346, U.S. Pat. No. 4,235,707, and U.S. Pat. No. 4,135,207.
Therefore, it is a principal object of the present invention to provide an improved method for increasing the chemical and biological reactivity of cellulose and/or hemicellulose in animal feedstuff materials. Another object is to provide a method having these attributes which is cost effective and readily applicable to a variety of feedstuff materials. A further object of this invention is to provide products which are readily adaptable and useful for a variety of purposes including the production of feedstuffs for ruminant animals and the production of raw materials for the production of fermentation products such as alcohols, and the like. This process can also be employed to produce cellulose-containing materials whose water holding capacity is increased. Other objects and advantages of the invention will become apparent from the following detailed descriptions of the preferred embodiments.